Method of coating a succession of aluminum surfaces



United StatCS PatentO METHOD OF COATING A SUCCESSION OF I ALUMINUM SURFACES No Drawing. Application September 24, 1958 Serial N0. 762,932

3 Claims. (21. 148-616) This invention relates to the art of applying a chemically bonded coating to the surface of aluminum or alloys thereof in which aluminum is the principal ingredient, and in the following disclosure the word aluminum is intended to include such alloys. More particularly, the in- 4' vention relates to that type of coating procedure which involves the use of an aqueuos acid bath which is applied to a succession or series of surfaces or pieces. This technique, as is well known, involves proper renewal or control of the composition of the bath, which composition will, of

course, vary as work is processed in the bath, and our invention is particularly concerned with maintaining the bath in proper operating condition.

Still more specifically, our invention relates to that type of coating procedure in which the surface of the aluminum is treated with aqueous acid coating solutions in which the essential coating producing ingredients are fluoride ions, hexavalent chromium ions, phosphate (P0 ions and hydrogen (H) ions. Such solutions are now very well known to the art and they produce decorative as well as corrosion resistant coatings on the aluminum, and typical examples of such processes are described in US. Patents 2,438,877; 2,472,864; 2,678,291 and 2,814,577.

As exemplified by the patents just mentioned the solutions employed have their chemical constituents present in amounts corresponding to the following: fluoride ions, 0.15 to 12.5 gms./liter; hexavalent chromium ions (expressed as CrO 3 to 60 gms./liter; and phosphate ions (P0 2 to 285 gms./liter.

Processes such as exemplified by the patents mentioned when the baths are maintained as described will produce excellent corrosion resistant coatings on aluminum surfaces even after the baths have been used for long periods of time. However, where it is desired to produce a suc cession of coatings of substantially uniform color and appearance it has been found necessary to replenish the coating baths in such a way as to maintain in them a constant level of fluoride ion activity regardless of the apparent total fluorine/CrO content of the bath. The method for accomplishing this latter is disclosed in Patent No. 2,814,577.

However, still another problem arises after the baths have been used for long periods of time, namely, that even though the coatings may be corrosion resistant and substantially uniform in color if proper precautions are taken, they tend to be overlayed with a powdery deposit. Such powdery coatings, of course, are many times objectionable.

With the foregoing history of the art in mind, it can be said that the principal object of our invention is to eliminate the formation of powdery coatings when produced by means of the familiar processes just described and panticularly exemplified in the patents referred to. A concomitant object of the invention is to keep the solutions employed substantially free of dissolved aluminum.

Laboratory work indicates that the chief causative factor in the production of powdery coatings is due to the Patented Oct. 20, 1959 presence in the bath of cations other than hydrogen and in an effort to remove these cations it has been suggested that cation exchange resins, either natural or synthetic, be employed and that the bath be passed through exchange units employing such resins. However, for some unexplained reason, while such treatment has proved eminently practical for the removal of cations such as sodium, potassium, trivalent chromium, ammonium, etc., it has not been successful in removing aluminum from the coating baths. In this connection it will be realized, of course, that aluminum accumulates in the coating baths as the bath is continued in service in the treatment of a succession of aluminum surfaces. When the content of aluminum in the bath approaches from 0.5 to 1 gm./liter the coatings produced by the baths tend to be powdery.

Our invention is based upon the discovery that the content ofdissolved aluminum which accumulates in the bath can be prevented from rising above the danger point of approximately 1 gram per liter if there be added to the bath at least 1 mol of sodium and 2 mols of potassium for every mol of aluminum dissolved from the work in the coating procedure. Indeed, if the bathshould accumulate more than 1" gram per liter of-dissolved aluminum, it can be precipitated therefrom by adding sutficient sodium and potassium in the molar ratio of 1 to 2 for each gram of aluminum found to be present. At the 'same time, as will be appreciated, the solution should be subjected to the action of the cation exchange resins mentioned and this can be accomplished by constantly withdrawing a portion of the bath during the processing procedure and passing it through the cation exchange resin and then returning it to the main body \of the bath. While any of the well known exchange resins are suitable for-the purpose, we have secured excellent results with a product ofthis nature produced by the Rohm & Haas Company as .Amberlite IR120, H form. This product is known to the art as a strongly acidic cation exchanger of the polystyrene nuclear sulfonic acid type. a

In adding the sodium and potassium as just described, it is preferred practice to introduce them into the bath in the form of one part sodium fluoride to two parts of potassium fluoride for each part of dissolved aluminum present in the bath because, by so doing, the baths will be maintained in suitable condition not only to produce tightly adherent and powder free coatings but also to yield uniformity of color in the finished coatings, because additions made in this way will maintain the bath substantially constant in fluoride activity which latter is the important consideration in maintaining uniformity of color;

We should say at this point that it is quite possible to add more than 1 mol of sodium and 2 mols of potassium to the bath for each mol of aluminum present because, by virtue of the employment of the cation exchange resin, any excess of sodium and potassium is absorbed in the resin so that there is no undesirable build-up of sodium and potassium ions in the bath.

In order to exemplify the improvement resulting from the process of the present invention we wish to refer to the following experimental work. We prepared two coating baths of 4 liters each, one of which baths we will refer to as bath A and the other as bath B. Each bath consisted of the following:

2.18 grams hydrofluoric acid per liter 14.2 grams chromic acid per liter 67 grams phosphoric acid per liter Etch rate, 45.4 mgm./ sq. ft. on half hour immersion time at F.

Both baths were heated to approximately 115 F. and a series of aluminum pieces were processed in each bath for a period of five minutes. During the early portion of this period both baths produced excellent coatings. Both baths were replenished in relation to their chromic and phosphoric acid content in the manner familiar to those skilled in this art. The fluoride ion concentration of bath A was maintained at .a substantially constant. figure by periodical additions of hydrofluoric acid. While the color of the coatings produced by bath A remained constant, after sq. ft. of aluminum had been processed, the coatings produced became extremely powdery.. Bath B was operated in a fashion identical to bath A but periodically bath B was analyzed for the presence of aluminum and the fluoride content was then replenished by adding to the bath 1 mol of sodium fluoride and 2 mols of potassium fluoride for every mol of dissolved aluminum found to be present. By proceeding in this way we found that bath B continued to operate satisfactorily, producing uniformly colored, powder free coatings even after 80 sq. ft. of aluminum had been processed in the bath. Etch rate measurements made periodicallyon bath B showed that the etch rate was maintained substantially constant.

After bath A had been used to treat 20 sq. ft. of surface area, a third experiment was made with this bath. It was analyzed for the presence of dissolved aluminum and was found to contain 2 gms./liter. By adding to the bath 3.12 grams of sodium fluoride and 8.6 grams of potassium fluoride a precipitate was formed and subsequent analysis of the bath indicated that the aluminum had been substantially completely removed. Etch rate measurments made before and after the addition of the sodium fluoride and the potassium fluoride showed that the [fluoride activity of the bath had not been increased by such additions and also that aluminum processed in the bath received excellent powder free coatings.

In running baths A and B as just described a portion of the bath was constantly withdrawn during the processing operations, passed through a cation exchange resin of the character described above and then returned to the bath.

We claim:

-1. In the art of coating a series of aluminum pieces 'wherein the surfaces thereof are treated successively with an aqueous acid bath the coating producing ingredients of which consist essentially of fluoride, hexavalent chromium, phosphate and hydrogen ions; the method of controlling the composition of the bath which comprises constantly withdrawing a portion of the bath, passing it through a cation exchange resin and returning it to the bath; maintaining the fluoride activity of the bath by periodically adding thereto at least '1 mol of sodium 5 fluoride and 2 mols of potassium fluoride for each mol of aluminum dissolved in the bath; and maintaining the hexavalent chromium, the phosphate and the hydrogen ion content by suitable additions as may be required.

2. A method for maintaining the content of dissolved aluminum in an aqueous, acid, aluminum coating bath of the type in which the coating producing ingredients consist essentially of fluoride, hexavalent chromium, phosphate and hydrogen ions, at less than 1 gram per liter, said method comprising adding to the bath 1 mol of sodium fluoride and ,2 mols of potassium fluoride for each mol of dissolved aluminum; constantly withdrawing a portion of the bath, passing it through a cation exchange resin and returning it to the bath; and adding hexavalent chromium, phosphate and hydrogen ions as required to maintain the bath in coating producing condition. V 3. In the art of coating a series of aluminum pieces wherein the surfaces thereof are treated successively with an aqueous acid bath the coating producing ingredicuts of which consist essentially of fluoride, hexavalent chromium, phosphate and hydrogen ions and wherein the said constituents are replenished as required to maintain the bath in coating condition; the method of controlling the content of cations other than hydrogen which comprises constantly withdrawing a portion of the bath, passing it through a cation exchange resin and returning it to the bath, and replenishing the fluoride ion content of the bath by adding thereto at least 1 mol of sodium fluoride and 2 mols of potassium fluoride for each mol of aluminum dissolved in the bath.

' References Cited in the file of this patent UNITED STATES PATENTS Spruance Mar. 30, 1948 2,494,908 Spruance et al Jan. 17, 1950 2,777,785 Schuster et a1. Jan. 15, 1957 I FOREIGN PATENTS 730, 897 Great Britain June 1, 1955 

1. IN THE ART OF COATING A SERIES OF ALUMINUM PIECES WHEREIN THE SURFACES THEREOF ARE TREATED SUCCESSIVELY WITH AN AQUEOUS ACID BATH THE COATING PRODUCING INGREDIENTS OF WHICH CONSIST ESSENTIALLY OF FLUORIDE, HEXAVALENT CHROMIUM, PHOSPHATE AND HYDROGEN IONS; THE METHOD OF CONTROLLING THE COMPOSITION OF THE BATH WHICH COMPRISES CONSTANTLY WITHDRAWING A PORTION OF THE BATH, PASSING IT THROUGH A CATION EXCHANGE RESIN AND RETURNING IT TO THE BATH; MAINTAINING THE FLUORIDE ACTIVITY OF THE BATH BY PERIODICALLY ADDING THERETO AT LEAST 1 MOL OF SODIUM FLUORIDE AND 2 MOLS OF POTASSIUM FLUORIDE FOR EACH MOL OF ALUMINUM DISSOLVED IN THE BATH; AND MAINTAINING THE HEXAVALENT CHROMIUM, THE PHOSPHATE AND THE HYDROGEN ION CONTENT BY SUITABLE ADDITIONS AS MAY BE REQUIRED. 